1. Field of the Invention
The present invention relates to an optical pickup apparatus performing a reading-out operation of a signal recorded in an optical disc or a recording operation of a signal in the optical disc.
2. Description of the Related Art
There has been widespread optical disc apparatuses capable of a signal reading-out operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of an optical disc.
FIG. 3 is a schematic diagram illustrating a general optical pickup apparatus used in an optical disc device. In this figure, reference numeral 1 denotes a laser diode emitting laser beam, which is red light with a wavelength of 650 nm, for example, and reference numeral 2 denotes a diffraction grating that the laser light emitted from the laser diode 1 enters and that includes a diffraction grating portion 2a for dividing the laser light into 0th order light, +1st order diffraction light and −1st order diffraction light and a half-wave plate 2b for converting the incident laser light to linear polarized light in an S direction.
Reference numeral 3 denotes a polarizing beam splitter that the laser light passing through the diffraction grating 2 enters and that includes a control film 3a reflecting S-polarized laser light and allowing the laser light polarized in a P direction to pass therethrough. Reference numeral 4 denotes a monitor photodetector provided at a position where the laser light having passed through the control film 3a included in the polarizing beam splitter 3 out of the laser light emitted from the laser diode 1 impinges on the photodetector, and detection output of the photodetector is used for controlling output of the laser light to be emitted from the laser diode 1.
Reference numeral 5 is a quarter-wave plate that is provided at a position where the laser light reflected at the control film 3a of the polarizing beam splitter 3 enters and that converts the incident laser light from linear polarized light into circular polarized light. Reference numeral 6 denotes a collimating lens that the diffused laser light having passed through the quarter-wave plate 5 enters, that converts the incident laser light into parallel light, and that corrects spherical aberration caused by a protective layer of an optical disc D.
Reference numeral 7 is a reflection mirror that the laser light having been converted into the parallel light by the collimating lens 6 impinges on and that reflects the laser light in a direction of a signal face of the optical disc D, and that return light reflected from a signal recording layer of the optical disc D impinges on and that reflects the return light in a direction of the polarizing beam splitter 3, as will be described later.
Reference numeral 8 is a sensor lens that the return light having passed through the control film 3a included in the polarizing beam splitter 3 enters, and includes a cylindrical face, a flat face, a concave face or a convex face and the like formed on the sides of the incident face and outgoing face. Such sensor lens 8 generates a focus error signal to be used for a focusing control operation by generating astigmatism in the return light.
Reference numeral 9 is a photodetector that is provided at a position where the return light having passed through the sensor lens 8 is focused and impinges thereon, and that is made up of a four-divided sensor and the like, in which photodiodes are arranged. The configuration of the photodetector 9 and the generation operation of the focus error signal by astigmatism method and the like are known and the description is omitted.
Reference numeral 10 is an objective lens that the laser light reflected by the reflection mirror 7 enters and that focuses the incident laser light to the signal recording layer included in the optical disc D. The objective lens 10 is fixed to a lens holder provided so as to be capable of a displacement operation in a direction perpendicular to the signal face of the optical disc, i.e., the focusing direction, as well as a radial direction of the optical disc, i.e., the tracking direction, by four support wires, for example. The configuration of such supporting mechanism of the lens holder and the like is known and the description is omitted.
In the case of performing a reproduction operation of a signal recorded in the optical disc D by using the optical pickup apparatus with the above configuration, a driving current is supplied to the laser diode 1, and laser light with a wavelength of 650 nm is emitted from the laser diode 1. The laser light emitted from the laser diode 1 enters the diffraction grating 2, where the laser light is divided by the diffraction grating portion 2a making up the diffraction grating 2 into the 0th order light, +1st order diffraction light, and −1st order diffraction light and converted by the half-wave plate 2b into the linear polarized light in the S direction. The laser light having passed through the diffraction grating 2 enters the polarizing beam splitter 3 and is reflected by the control film 3a included in the polarizing beam splitter 3, while a part of the laser light passes through the control film 3a to be applied to the monitor photodetector 4.
The laser light reflected by the control film 3a enters the collimating lens 6 through the quarter-wave plate 5, and is converted by the collimating lens 6 into the parallel light. The laser light converted by the collimating lens 6 into the parallel light is reflected by the reflection mirror 7, to enter the objective lens 10. The laser light incident on the objective lens 10 is applied to the signal recording layer of the optical disc D as a spot by a focusing operation of the objective lens 10. The laser light emitted from the laser diode 1 is applied as a desired spot to the signal recording layer of the optical disc D as described above, and in this case, the numerical aperture of the objective lens 10 is set at 0.6.
When the above-mentioned focusing operation of the laser light is performed by the objective lens 10, spherical aberration is generated due to a difference in thickness of the protective layer between the signal recording layer and the signal incident face of the optical disc D, however, adjustment can be made so as to minimize the spherical aberration by displacing the collimating lens 6, shown in an embodiment according to the present invention, in an optical path direction. Such an adjustment operation is commonly performed, and the description is omitted.
An irradiation operation of the laser light to the signal recording layer included in the optical disc D is performed by the above-mentioned operation, and when such an irradiation operation is performed, the return light reflected from the signal recording layer enters the objective lens 10 from the side of the optical disc D. The return light incident on the objective lens 10 enters the polarizing beam splitter 3 through the reflection mirror 7, collimating lens 6, and quarter-wave plate 5. Since the return light incident on the polarizing beam splitter 3 has been converted by the quarter-wave plate 5 into the linear polarized light in the P direction, thereby passing through the control film 3a provided in the polarizing beam splitter 3.
The return light of the laser light having passed through the control film 3a enters the sensor lens 8, and astigmatism is generated due to a function of the sensor lens 8. The return light in which the astigmatism has been generated due to the sensor lens 8 is applied to a sensor portion such as the four-divided sensor provided in the photodetector 9 by the focusing operation of the sensor lens 8. As the result of the irradiation of the return light to the photodetector 9 as above, a generating operation of a focus error signal is performed as known by using change in shape of a spot formed by irradiation on the sensor portion included in the photodetector 9. With use of such a focus error signal, the objective lens 10 is displaced in a direction of the signal face of the optical disc D, and thus, the focus control operation can be performed.
Although a description is omitted, a configuration is made so as to be able to perform a known tracking control operation with the use of the +1st order diffraction light and the −1st order diffraction light generated by the diffraction grating 2, and such a tracking control operation is performed to displace the objective lens 10 in the tracking direction, and thus, the reading-out operation of a signal recorded in the optical disc D is performed.
The reading-out operation of a signal recorded in the optical disc D is performed as mentioned above, and while the reading-out operation is performed, a part of the laser light is applied to the monitor photodetector 4, and a value of the driving current to be supplied to the laser diode 1 can be controlled by using change in level of a monitor signal obtained from the monitor photodetector 4.
Since the output of the laser light can be controlled by controlling the valued of the driving current to be supplied to the laser diode 1, the adjustment operation can be performed of the laser output required when performing not only the reproduction operation of a signal recorded in the optical disc D but also the recording operation of a signal in the optical disc D can be performed.
A general optical pickup apparatus is configured as shown in FIG. 3, and as obvious from the figure, a configuration is made such that the laser light converted by the collimating lens 6 into the parallel light is reflected in the direction of the objective lens 10 by the reflection mirror 7 and the reflection direction is at right angles to the parallel light.
An arrangement is made such that the optical axis of the objective lens 10 is perpendicular to the signal face of the optical disc D and the laser light converted by the collimating lens 6 into the parallel light is at right angles to the optical axis of the objective lens 10. As a result, a configuration is made such that a mounting angle of the reflection mirror 7 with respect to a base 11 is 45 degrees as shown in FIG. 3.
In FIG. 3, optical elements such as the polarizing beam splitter 3 and collimating lens 6 are illustrated differently in size and arrangement from those in practice for explanation's sake. That is, a configuration of an optical pickup apparatus in practice is made so as to arrange all the optical elements in a space portion between the base 11 and the optical disc D.
The thickness of the optical pickup apparatus, that is, the distance between the base 11 and the disc D is determined by the height of the reflection mirror 7, the distance between the reflection mirror 7 and the objective lens 10, the thickness of the objective lens 10, and the distance between the objective lens 10 and the optical disc D.
In optical pickup apparatuses with the same type of objective lenses, since the distance between the reflection mirror 7 and the objective lens 10 and the distance between the objective lens 10 and the optical disc D can not be reduced among the elements determining the thickness of the optical pickup apparatus, the height of the reflection mirror 7 is required to be reduced.
If the mounting angle of the reflection mirror 7 with respect to the base 11 is set at 45 degrees, the height of the reflection mirror 7 is required to be greater than the diameter of the laser light, and thus, the height of the reflection mirror 7 can not be reduced.
As a method for reducing the thickness of an optical pickup apparatus and solving such problem, there is proposed an art using a prism (See Japanese Patent Laid-Open Publication No. 2000-113492).
In the above-mentioned Japanese Patent Laid-Open Publication No. 2000-113492, there is described an optical pickup apparatus using a prism as means for changing a light path so that laser light having been converted by the collimating lens into parallel light is led in a direction of an objective lens.
According to such an art, the optical pickup apparatus can be reduced in thickness, however, the laser light is reflected only once on each reflection face provided in the prism to be led in the objective lens direction, so that further reduction of the thickness has not been possible.